Analysis of the Bi-modal Nature of Solar Wind { Magnetosphere Coupling

It has been shown that the optimal linear prediction lter relating the solar wind electric eld and the geomagnetic activity, as measured by the AL index, is both bi-modal and dependent on the level of activity in the magnetosphere. Further studies truncated the prediction lter to a ve parameter model containing two low-pass ltered delta functions of arbitrary amplitude and delay time. The present study elaborates on the nature of the bi-modal response by using the ve parameter model to quantify the e ects of the level of geomagnetic activity on each of the modes of the lter individually. We nd that at all levels of activity, the second mode, occurring at approximately one hour, is relatively unchanged. The rst mode, however, has a necessary one parameter dependence on the level of activity in the magnetosphere. The amplitude of the rst mode is shown to increase with respect to activity, and this dependence is su cient in characterizing the changing properties of the magnetosphere with respect to activity.